Ops on a tiny ship like Voyager is a job for an ensign, which is why they gave it to an ensign.
Ops on Enterprise, which is almost 10 times the size of Voyager, and expected to do anything, anywhere (Ops is also about resource management of the ships stores. The Ops officer is the quartermaster as well as traffic controller for everything else.) is not a job for an ensign green straight out of the academy, even though there were extras in those seats all the time.
This isn't a question of just sitting at the Ops station.
This is about the person, Harry or whoever, being a section head.
Commanding staff responsibly.
How much staff, floaters and fixed do you think these two very different Section heads of Ops have to roster and organize on there respective ships?
3 or 4 shifts a day depending on what era we're talking about, then redundancies they can put into play and then green buggers they're still training up.
Data probably had 20 to thirty lieutenants in his stable of Ops officers at any one point. Kim on the other hand probably had 4 or 5 enlisted personal to push around in his ops department because no one above the rank of Ensign during the original "mission" can (without a grain of salt) work in ops on Voyager because Kim can't give them orders or write crew evaluations on his superiors recommending advancement or demotion or transfer, unless you believe that he has magic rank from being a section head like how O'Brien does.
besides if Kim had a huge staff, then why do we almost never see anyone else sitting at the ops station unless he wanders off to do something important like an away mission?